Lactoferrin is a glycoprotein having a molecular weight of about 80,000, which occurs mainly in mammalian milk and is also found in neutrophils, tears, saliva, nasal discharge, bile, semen, etc. Because of its iron-binding ability, lactoferrin belongs to the transferrin family. Known physiological activities of lactoferrin include an antibacterial effect, an iron metabolism regulatory effect, a cell proliferation promotion effect, a hematopoietic effect, an anti-inflammatory effect, an antioxidative effect, a phagocytosis enhancement effect, an antiviral effect, a bifidobacteria growth promotion effect, an anticancer effect, a cancer metastasis inhibitory effect, a translocation inhibitory effect and so on. Further, recent studies have indicated that lactoferrin also has a lipid metabolism improvement effect, an analgesic or anti-stress effect, and an anti-aging effect. As described above, lactoferrin is a multifunctional physiologically active protein having a wide range of functions and is therefore expected for use in, e.g., pharmaceutical and/or food applications for the purpose of restoration or promotion of health. Food products containing lactoferrin have already been commercially available.
When given orally, lactoferrin will be hydrolyzed by the action of pepsin, an acidic protease contained in the gastric juice, and then cleaved into peptides. For this reason, lactoferrin molecules are almost unable to reach the intestinal tract. However, lactoferrin receptors are known to be present on the small intestinal mucosa in the case of the digestive tract, and recent studies have indicated that lactoferrin is taken into the body through the intestinal tract and exerts its biological activities. Thus, for exertion of the lactoferrin's biological activities, it is important to ensure that lactoferrin is allowed to reach the intestinal tract without being hydrolyzed by the action of pepsin in the gastric juice. Moreover, when formulated into injections, lactoferrin will be exposed to cleavage catalyzed by proteases (e.g., chymotrypsin, elastase) contained in tissues and organs, so that it is practically important to impart resistance against these proteases for the purpose of increasing the in vivo stability in tissues and organs where lactoferrin is administered.
IgG antibodies are known to have a long half-life in blood because they are prevented from being cleaved in vivo through a recycling mechanism mediated by the neonatal Fc receptor (hereinafter referred to as “FcRn”). In addition, antibody drugs, whose targets are limited to specific proteins or peptides or the like and whose mechanisms of action are therefore limited, are regarded as having fewer side effects than conventional synthetic compounds.
The concept of biological formulations based on fusion proteins with IgG antibody or its Fc region has been known per se. By way of example, as an agent for suppressing acute graft rejection following renal transplantation, a CD3-targeting antibody drug was approved in 1986 and has been used over a long period of time.
However, in general, fusion proteins are often observed to have reduced biological activities when compared to non-fused proteins. This is because their active sites have been affected as a result of fusion. By way of example, when compared to endogenous TPO, TPOR-binding mimetic peptides were found to be comparable in terms of TPOR-binding levels, but tended to have slightly lower biological activities when tested in vitro.
Furthermore, in the case of a fusion protein formed with IFN-α and Fc region, which is designed to increase the half-life of IFN-α in blood, the half-life in blood has been greatly increased but there arises a disadvantage in that the physiological activities of IFN-α are reduced. Thus, with regard to conditions and others required for preparation of fusion proteins having desired properties, sufficient studies should be conducted for each protein.